Polymerisation processes for the manufacture of polymer products of anionic polymerisation, such as lithium butadiene rubber, styrene-butadiene rubber and butadiene-styrene-isoprene rubber are well known. It is also well known that stabilisers, such as antioxidants, may be added to the polymer cement following polymerisation to reduce the natural degradation and resulting discolouration of the polymer. The desire to sustain colourless product is particularly important to butadiene rubber or styrene-butadiene rubber in HIPS (high impact polystyrene) applications. A commonly used antioxidant system in the manufacture of lithium-butadiene rubber, for example, is tris(nonylphenol) phosphite (TNPP) in combination with phenolic antioxidants, such as Irganox 1076. However, as part of efforts to improve the manufacturing processes to reduce or eliminate health hazardous materials, alternative antioxidant systems are being sought and tested. It is known that Irganox 1520, a hindered phenol, provides desirable properties in the polymer and reduces degradation to a degree comparable to current antioxidant systems. However, it has been recognized in the past (U.S. Pat. No. 3,658,743) that hindered phenols used as antioxidant may cause discolouration. Byproducts of low purity Irganox 1520 can form colour bodies during the recovery process, which can result in a coloured rubber product. Polymer treated and recovered using low purity grades of Irganox 1520 alone does not provide a product which remains colourless after thermal aging (herein referred to as "sustained colourlessness").
Iganox 1520 is available in three grades, which differ in degree of purity. Energy consuming distillation is used to purify or reduce impurities in the crude Irganox 1520 product to remove higher and lower boiling byproducts. A product with a relatively lower proportion of byproducts (such as odour causing mercaptans) was available as Irganox 1520 L grade. A product with relatively higher proportion of byproducts has been offered as Irganox 1520 D and may be further purified to produce Irganox 1520 L. The Irganox 1520 L grade is additionally reacted with epoxidised soya bean oil for further removal of byproducts such as mercaptans to reduce odour. This product is available as Irganox 1520 LR grade.